1. Field of the Invention
The present invention relates to a zoom lens system suitable for, for instance, image-taking apparatuses such as video cameras, digital still cameras and silver halide cameras.
2. Description of the Related Art
Recently, remarkable advancement of the miniaturization of zoom lenses for image-taking, which is associated with the miniaturization and weight reduction of home video cameras, etc., is seen, and especially, shortening the total length of the lens system, miniaturizing the front lens diameter and simplifying the lens composition are attempted.
As one of means to achieve these purposes, a so-called rear focus type zoom lens is known, in which a lens unit other than the first lens unit on the object side is moved for focusing.
Generally, in the rear focus type zoom lens, an effective diameter of the first lens unit is small as compared with a zoom lens in which the first lens unit is moved for focusing, and it is easy to miniaturize the entire system of the rear focus type zoom lens. Moreover, close image-taking, especially, very close image-taking becomes easy. In addition, there are features such as a possibility of rapid focusing, etc. since the driving power of the lens unit can be small by the miniaturization and weight reduction of the lens unit.
Conventional zoom lenses of the rear focus type have four lens units comprising, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power. In the zoom lenses, the second lens unit is moved for varying the magnification, and the fourth lens unit is moved for compensating the image plane fluctuation associated with the variation of magnification and for focusing (see Patent Documents 1 and 2 described below).
Moreover, zoom lenses are known, which have four lens units comprising, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, wherein the second, third and fourth lens units are moved for zooming (see Patent Document 3 described below).
Moreover, zoom lenses are known, which have four lens units comprising, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, wherein the first, second, third and fourth lens units are moved for zooming (see Patent Document 4 described below).
Furthermore, zoom lenses are known, which have four lens units comprising, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, wherein the first, second, third and fourth lens units are moved for zooming, and the miniaturization of the total length of the zoom lens and the reduction of the front lens diameter are attempted by independently moving the aperture stop (see Patent Document 5 described below).
[Patent Document 1]
Japanese Laid-Open Patent Application No. H07-270684 (corresponding to U.S. Pat. No. 5,963,378)
[Patent Document 2]
Japanese Laid-Open Patent Application No. H11-305124 (corresponding to U.S. Pat. No. 6,166,864)
[Patent Document 3]
Japanese Patent No. 2879463 (corresponding to U.S. Pat. No. 5,179,472)
[Patent Document 4]
Japanese Laid-Open Patent Application No. 2001-194586 (corresponding to U.S. Pat. No. 6,456,441)
[Patent Document 5]
Japanese Patent No. 3352240
In the zoom lens disclosed by Patent Document 3, it is difficult to miniaturize the front lens diameter since the movement track of the third lens unit for zooming is a track convex towards the image side.
Moreover, in the zoom lens disclosed by Patent Document 4, it is difficult to obtain a high zoom ratio since the first lens unit is a single lens, and the refractive power of the first and second lenses are not so large.
Moreover, in the zoom lens disclosed by Patent Document 5, it is difficult to miniaturize the front lens diameter since the movement tracks of lens units for zooming are not necessarily the best.
In general, in a case where the sufficient light quantity in the peripheral area of the image plane is secured in four-unit-zoom lenses constituted by a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a positive refractive power and a stop arranged near the third lens unit, there is little decrease in the peripheral light quantity in an area up to about the 80 percent height position of the peripheral area in the image plane, but there is a steep decrease in the peripheral light quantity in an outer area (peripheral portion) from about the 80 percent height position in the image plane. Therefore, there is a problem that the decrease of the peripheral light quantity stands out.